The melanosome is the unique organelle of the vertebrate pigment cell, the site of synthesis and deposition of melanin pigment. The genetics of vertebrate pigmentation have been studied extensively, especially in the mouse where a set of inbred pigmentation mutants with common genetic background is available. The goal of this project is to develop a biochemical anatomy of the melanosome especially in terms of its protein subunits and then to study the synthesis and assembly of these proteins for form melanosomes and finally to study the control of melanosome formation by some of the genetic loci which determine melanosome structure. Immunological, electrophoretic, and enzymatic analysis is currently being used to study melanosomes from two serially transplanted mouse melanotic tumors, black melanoma B-16 and brown melanoma S-91 which are genetically different from one another at the B locus. Tyrosinase, the enzyme of melanin synthesis is assayed by measuring the hydroxylation of tyrosine to DOPA by the method of Pomerantz. Melanosome protein components are separated and resolved by acrylamide gel electrophoresis and immunofluorescent assays for the unique melanosome antigens will be developed. These biochemical criteria for pigment cell differentiation will be used to study genetic and epigenetic factors controlling the development of pigment cell phenotype in cell cultures and in embryos.